Spring GeneratedValue annotation usage

Question

Let's say that I have a Car entity:

@Entity 
public class Car {
      @Id
      @GeneratedValue(strategy=GenerationType.AUTO)
      private Integer id;

How does spring know what value to autoincrement when I add a new object to the database?

Answer 1

Here is a good explanation of primary keys generation strategies

There are 4 options to generate primary keys

GenerationType.AUTO

The GenerationType.AUTO is the default generation type and lets the persistence provider choose the generation strategy.

If you use Hibernate as your persistence provider, it selects a generation strategy based on the database specific dialect. For most popular databases, it selects GenerationType.SEQUENCE.

GenerationType.IDENTITY

The GenerationType.IDENTITY is the easiest to use but not the best one from a performance point of view. It relies on an auto-incremented database column and lets the database generate a new value with each insert operation. From a database point of view, this is very efficient because the auto-increment columns are highly optimized, and it doesn’t require any additional statements.

This approach has a significant drawback if you use Hibernate. Hibernate requires a primary key value for each managed entity and therefore has to perform the insert statement immediately. This prevents it from using different optimization techniques like JDBC batching.

GenerationType.SEQUENCE

The GenerationType.SEQUENCE uses a database sequence to generate unique values. It requires additional select statements to get the next value from a database sequence. But this has no performance impact for most applications.

If you don’t provide any additional information, Hibernate will request the next value from its default sequence. You can change that by referencing the name of a @SequenceGenerator in the generator attribute of the @GeneratedValue annotation. The @SequenceGenerator annotation lets you define the name of the generator, the name, and schema of the database sequence and the allocation size of the sequence.

@Id
@GeneratedValue(strategy=GenerationType.AUTO)
@SequenceGenerator(name="car_generator", sequenceName = "car_seq", allocationSize=50)
private Long id;

(Side note: Usually prefer Long for ids instead of Integer so you're less likely to run out)

GenerationType.TABLE

The GenerationType.TABLE gets only rarely used nowadays. It simulates a sequence by storing and updating its current value in a database table which requires the use of pessimistic locks which put all transactions into a sequential order. This slows down your application, and you should, therefore, prefer the GenerationType.SEQUENCE, if your database supports sequences, which most popular databases do.